CHAPTER VI 



CERATODUS, A CHORD ATE WITH A LUNG 



Externals. — Ceratodus is the Australian lung-fish, a group of 

 great importance, whose only other living representatives are 

 Protopterus in Africa and Lepidosiren in South America. It 

 is not an uncommon thing for ancient and primitive groups of 

 animals to have gone extinct everywhere except for definite, 

 small, and isolated regions of the earth. These animals are 

 therefore an example of discontinuous geographical distribution. 

 In shape, Ceratodus is typically a fish. The median fins 



) ) 



) ) > ) > } ) ' ) ' i 



. ; ) ; > ) > > 



r »x>' 



tf. 



Fig. 39. — Ceratodus : view from the left side. (Partly after Goodrich.) 



e, eye ; pc and pi, pectoral and pelvic " archipterygial " fins ; tf, tail- 

 fin (diphy cereal ?). 



are remarkable in that the dorsal, caudal, and ventral fins are 

 all continuous with one another. Further, the tail-fin is 

 symmetrical and pointed, and resembles the primitive di- 

 phy cereal type, such as is found in Cyclostomes. There is, 

 however, a certain amount of doubt as to whether the tail-fin 

 of Ceratodus is primitive. When a tail such as this is derived 

 secondarily by simplification from another type (as is the case 

 in the eel, for example) it is called gephyrocercal. 



The paired fins are elongated and leaf- like. They have a 



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